Structural studies of ReO(V) mixed ligand [SNS][Cl] and [SNS][S] complexes

Article abstract of DOI:10.1016/S0020-1693(98)00422-8

The synthesis, characterization and spectroscopic properties of four oxorhenium complexes, ReO[C₂H₅SCH₂CH₂N(CH ₂CH₂S)₂]Cl (1), ReO[C₂H₅SCH₂CH₂N(CH ₂CH₂S)₂][p-CH₃OC₆H ₄S] (2), ReO[(C₂H₅)₂NCH₂CH ₂N(CH₂CH₂S)₂][p-CH₃C ₆H₄S] (3), and ReO[C₅H₁₀NCH₂CH₂N(CH ₂C(CH₃)₂S)₂][C₆H ₅CH₂S] (4) are reported. These neutral and lipophilic complexes are designed according to the '3+1' mixed ligand concept. X-ray crystallographic studies show that the coordination geometry around rhenium is distorted trigonal bipyramidal in complexes 2 and 3 and trigonally distorted square pyramidal in complex 4 (C₁₅H₂₄NO₂S₄Re (2), triclinic P1, a=9.610(2), b=9.628(2), c=11.791(2) A, α=108.307(8), β=67.735(6), γ=90.166(8)°, V=950.04 A³, Z=2; C₁₇H₂₉N₂OS₃Re (3), monoclinic P2₁/c, a=13.188(1), b=7.542(1), c=21.193(1) A, β=103.260(2)°, V=2051.74 A³, Z=4; C₂₂H₃₇N₂OS₃Re · 0.5C₂H₅OH (4), monoclinic P2₁/c, a=16.720(6), b=11.013(3), c=16.747(5) A, β=113.44(1)°, V=2829.08 A³, Z=4). Complete assignments of ¹H and ¹³C NMR resonances were made for all complexes and compared to those of analogous technetium complexes. Complex 3 was also prepared at the tracer ¹⁸⁶Re level.

Full text of DOI:10.1016/S0020-1693(98)00422-8

Inorganica Chimica Acta 287 (1999) 142–151
Structural studies of ReO(V) mixed ligand [SNS][Cl] and [SNS][S]
complexes
M. Pelecanou a,*, I.C. Pirmettis b, M.S. Papadopoulos b, C.P. Raptopoulou c,
A. Terzis c, E. Chiotellis b, C.I. Stassinopoulou a
a Institute of Biology, NCSR Demokritos, POB 60228, GR-153 10 Athens, Greece
b Institute of Radioisotopes-Radiodiagnostic Products, NCSR Demokritos, POB 60228, GR-153 10 Athens, Greece
c Institute of Materials Science, NCSR Demokritos, POB 60228, GR-153 10 Athens, Greece
Received 3 September 1998; accepted 18 November 1998
Abstract
The synthesis, characterization and spectroscopic properties of four oxorhenium complexes, ReO[C2H5SCH2CH2N(CH2-
CH2S)2]Cl (1), ReO[C2H5SCH2CH2N(CH2CH2S)2][p-CH3OC6H4S] (2), ReO[(C2H5)2NCH2CH2N(CH2CH2S)2][p-CH3C6H4S] (3),
and ReO[C5H10NCH2CH2N(CH2C(CH3)2S)2][C6H5CH2S] (4) are reported. These neutral and lipophilic complexes are designed
according to the ‘3+1’ mixed ligand concept. X-ray crystallographic studies show that the coordination geometry around
rhenium is distorted trigonal bipyramidal in complexes 2 and 3 and trigonally distorted square pyramidal in complex 4
˚
(
(C15H24NO2S4Re (2), triclinic P1, a=9.610(2), b=9.628(2), c=11.791(2) A, h=108.307(8), i=67.735(6), k=90.166(8)°,
3
˚
˚
V=950.04 A , Z=2; C17H29N2OS3Re (3), monoclinic P21/c, a=13.188(1), b=7.542(1), c=21.193(1) A, i=103.260(2)°,
3
˚
˚
V=2051.74 A , Z=4; C22H37N2OS3Re · 0.5C2H5OH (4), monoclinic P21/c, a=16.720(6), b=11.013(3), c=16.747(5) A,
i=113.44(1)°, V=2829.08 A , Z=4). Complete assignments of H and 13C NMR resonances were made for all complexes and
compared to those of analogous technetium complexes. Complex 3 was also prepared at the tracer 186Re level. © 1999 Elsevier
Science S.A. All rights reserved.
3
1
˚
Keywords: Crystal structures; Oxorhenium complexes; Mixed ligand complexes; NMR spectroscopy
1. Introduction
which causes the two elements to have almost identical
radii [1b,c,2]. As a consequence, analogous Tc and Re
complexes are expected to have similar physical proper-
ties (size, shape, dipole moment, formal charge, lipo-
The relationship between technetium and rhenium
coordination chemistry is the focus of much activity
because of the importance of the isotopes 99mTc, 186Re
and 188Re in diagnostic and therapeutic nuclear
medicine. 99mTc labeled pharmaceuticals comprise over
80% of the diagnostic agents used in clinics for imaging
of the body. 186Re and 188Re labeled pharmaceuticals
are being developed for radiotherapy applications with
the scope of delivering therapeutically significant radia-
tion doses to malignant lesions [1].
philicity, etc.) and to be indistinguishable by
a
biological system. On this basis, matched pairs of diag-
nostic 99mTc and therapeutic 186Re and/or 188Re radio-
pharmaceutical complexes are being synthesized and
studied. On the other hand, analogous technetium and
rhenium complexes present a number of predictable
differences in their chemical properties that may
provide a basis for biological distinction between ana-
logues. For example, rhenium complexes are more
difficult to reduce [1b,c,3] and more substitution inert
[4] than their technetium analogues. Differences in spec-
troscopic parameters between analogous Tc and Re
complexes are also widely reported in the literature and
are often related to the general chemistry of second-
Technetium and rhenium display similarity in their
properties which arises from their position in the peri-
odic table combined with the lanthanide contraction
* Corresponding author. Tel.: +30-1-650 3555; fax: +30-1-651
1767.
E-mail address: pelmar@mail.demokritos.gr (M. Pelecanou)
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00422-8

M. Pelecanou et al. / Inorganica Chimica Acta 287 (1999) 142–151
143
and third-row transition metals [5]. Recently [6], the
dependence of ligand NMR chemical shifts on the
identity of the metal center in analogous technetium
and rhenium complexes has been examined for a series
of hexacoordinated mono-oxo and di-oxo M(V) and
M(III) complexes with the aim of linking qualitatively
NMR chemical shift differences to known chemical
properties.
We wish to report here on the synthesis and spectro-
scopic properties of four new oxorhenium complexes,
ReO[C2H5SCH2CH2N(CH2CH2S)2]Cl (1), ReO[C2H5-
SCH2CH2N(CH2CH2S)2][p-CH3OC6H4S] (2), ReO-
[(C2H5)2NCH2CH2N(CH2CH2S)2][p-CH3C6H4S](3),and
ReO[C5H10NCH2CH2N(CH2C(CH3)2S)2][C6H5CH2S]
(4) prepared with the 187Re/185Re mixture of stable
rhenium isotopes. The synthesis of complex 3 at tracer
186Re level is also reported. These complexes belong to
the class of [SNS][S] [7] ‘3+1’ mixed ligand complexes
[8]. The analogues to 2 and 3 99mTcO complexes have
shown high brain uptake and significant retention in
animals tested [7a,g]. Comparisons of spectroscopic and
electrochemical properties of 1–4 to their technetium
analogues 1%–4%, are made wherever possible with the
aim of enriching the existing correlation data on rhe-
nium and technetium chemical properties.
as described previously [9]. The tridentate ligand 2,6-
dimethyl-4-(2-piperidin-1-ylethyl)-4-aza-2,6-heptanedi-
thiol was prepared according to the published proce-
dure [7c] by reacting 1-(2-aminoethyl)piperidine with
2,2%-dithiobis(2-methylpropanal) in the presence of
sodium cyanohydroborate followed by reduction with
Na/NH3.
The complex ReOCl3(PPh3)2 was prepared according
to literature methods [10] while the precursor and
Re(O)-citrate were generated in situ by known methods
[11] as described in detail below.
The 186ReO4− salt was
a generous gift from
Mallinckrodt Medical BV, Petten, Netherlands.
2.2. Chloro[N,N-bis(2-mercaptoethyl)-2-(ethylthio)-
ethylamine]oxorhenium(V) (1)
To a stirred suspension of trichlorobis(triphenylphos-
phine)rhenium(V) oxide (166 mg, 0.2 mmol) in
methanol (10 ml) a 1 N CH3COONa solution in
methanol (2 ml, 2 mmol) was added. N,N-Bis(2-mer-
captoethyl)-2-(ethylthio)ethylamine (45 mg, 0.2 mmol)
was added subsequently and the mixture was refluxed
until the green–yellow color of the precursor turned to
green. After being cooled to room temperature (r.t.),
the reaction mixture was diluted with CH2Cl2 (30 ml)
and then washed with water. The organic layer was
separated from the mixture and dried over MgSO4. The
volume of the solution was reduced to 5 ml and then 5
ml of methanol were added. Green crystals were formed
by slow evaporation. Yield 35%. FT-IR (cm−1, KBr
pellet): 964 (ReꢀO). Anal. Calc. for C8H17NOS3ClRe:
C, 20.84; H, 3.72; N, 3.04; S, 20.86. Found: C, 21.14;
H, 3.67; N, 2.71; S, 19.72%.
Technetium complexes 1%–4% (prepared with the long-
life 99Tc isotope) are available in our laboratory from
previous work, and already reported in the literature
[7a,c,f].
2. Experimental
2.1. Synthesis
Rhenium-186 is a b- and g-emitter and special pre-
cautions should be taken.
2.3. [(4-Methoxy)thiophenolato][N,N-bis(2-mercapto-
ethyl)-2-(ethylthio)-ethylamine]oxorhenium(V) (2)
All laboratory chemicals were of reagent grade. Ben-
zylmercaptan, 4-methylthiophenol and 4-methoxythio-
phenol used as coligands were purchased from Fluka.
IR spectra were recorded as KBr pellets on a Perkin-
Elmer 1600 FT-IR spectrophotometer and were refer-
enced to polystyrene. Elemental analyses were per-
formed on a Perkin-Elmer 2400/II automatic analyzer.
High performance liquid chromatography (HPLC)
analysis was performed on a Waters chromatograph
equipped with a 600E delivery system and a m-Bonda-
pak C-18 column using 85% methanol as the mobile
phase at a 1.0 ml min−1 flow rate. Rhenium complexes
were detected by a Waters 991 PDA photodiode array
detector and a Beckman 171 radioisotope detector.
The tridentate ligands N,N-bis(2-mercaptoethyl)-2-
(ethylthio)ethylamine and N,N-bis(2-mercaptoethyl)-
N%,N%-diethylethylenediamine were synthesized by
reacting ethylene sulfide with 2-(ethylthio)ethylamine or
N,N-diethylethylenediamine in an autoclave at 110°C
2.3.1. Method 1
To a stirred suspension of trichlorobis(triphenylphos-
phine)rhenium(V) oxide (166 mg, 0.2 mmol) in
methanol (10 ml) a 1 N CH3COONa in methanol (2 ml,
2 mmol) was added. A mixture of N,N-bis(2-mercap-
toethyl)-2-(ethylthio)ethylamine (45 mg, 0.2 mmol) and
28 mg (0.2 mmol) of 4-methoxythiophenol were added
under stirring. The solution was refluxed until the
green–yellow color of the precursor turned dark green.
After being cooled to r.t., the reaction mixture was
diluted with CH2Cl2 (30 ml) and then washed with
water. The organic layer was separated from the mix-
ture and dried over MgSO4. The volume of the solution
was reduced to 5 ml and then 5 ml of methanol were
added. Green crystals were formed by slow evapora-
tion. Yield 50%. FT-IR (cm−1, KBr pellet): 945
(ReꢀO), 1236 (Ar–O–CH3), 832 (arom). Anal. Calc. for

144
M. Pelecanou et al. / Inorganica Chimica Acta 287 (1999) 142–151
C15H24NO2S4Re: C, 31.90; H, 4.28; N, 2.48; S, 22.71.
Found: C, 31.62; H, 4.12; N, 2.35; S, 22.47%.
was agitated in a vortex mixer and left to react at r.t.
for 10 min. The pH of the reaction mixture was ad-
justed to 9 with 0.5 N NaOH and the aqueous phase
was extracted with two successive 1.5 ml portions of
CH2Cl2. The combined organic extracts were dried over
MgSO4 and filtered. Analysis of the mixture performed
by reversed phase HPLC (methanol:water, 85:15, flow
rate 1 ml min−1) showed a major radioactive peak
(\95%) with a retention time similar to that of com-
plex 3. The identity of 186Re-complexes was determined
by comparative HPLC studies using, as reference sam-
ples, the complexes isolated at carrier level.
2.3.2. Method 2
To a solution of SnCl2 (38.9 mg, 0.2 mmol) in 5 ml
of 0.5 M citric acid in water KReO4 (57.8 mg, 0.2
mmol) was added. The solution of ReO-citrate was
subsequently added to a mixture of N,N-bis(2-mercap-
toethyl)-2-(ethylthio)ethylamine (45 mg, 0.2 mmol) and
4-methoxythiophenol (28 mg, 0.2 mmol) and stirred at
r.t. for 30 min. The pH was adjusted to 9 with 0.5 N
NaOH and the mixture was extracted with CH2Cl2 and
dried over MgSO4. Ethanol was added to the solution
and after slow evaporation at r.t. green crystals precip-
itated and were collected by filtration. Yield 45%. Ana-
lytical data as above.
2.7. Crystallography
A green crystal of 2 (0.15×0.22×0.50 mm), a green
crystal of 3 (0.10×0.20×0.45 mm) and a green–grey
crystal of 4 (0.10×0.15×0.40 mm) were mounted in
air. Diffraction measurements were made on a P21
Nicolet diffractometer upgraded by Crystal Logic using
Zr-filtered Mo radiation. Unit cell dimensions were
determined and refined using the angular settings of 25
automatically centered reflections in the range 11B
2qB23o and are shown in Table 1. Intensity data were
recorded using a q−2q scan. Three standard reflec-
tions monitored every 97 reflections showed less than
3% variation and no decay. Lorentz, polarization and
C-scan absorption corrections were applied using Crys-
tal Logic software. The structures were solved by direct
methods using SHELXS-86 [12] and refined by full-ma-
trix least-squares techniques with SHELX-76 [13].
2.4. [(4-Methyl)thiophenolato][N,N-bis(2-mercapto-
ethyl)N%,N%-diethyl-ethylenediamine]oxorhenium(V) (3)
The complex was prepared as described above, using
either the Re(V)O-citrate or ReOCl3(PPh3)2 precursor
(0.2 mmol) and equimolar quantities (0.2 mmol) of
N,N - bis - (2 - mercaptoethyl) - N%,N% - diethylethylenedi-
amine and 4-methylthiophenol. The complex was iso-
lated as dark green crystals from a dichloromethane–
petroleum ether (40–60°C) solution. Yield 45%. FT-IR
(cm−1, KBr pellet): 939 (ReꢀO), 807 (arom) UV–Vis
(nm) 230, 294, 368, 422. Anal. Calc. for
C17H29N2OS3Re: C, 36.47; H, 5.22; N, 5.00; S, 17.18.
Found: C, 36.29; H, 4.93; N, 5.29; S, 17.16%.
Complex 2: 2qmax=52°, scan speed 4.5° per minute,
scan range 2.5+a1a2 separation, reflections collected/
2.5. [Benzylthiolato][N,N-bis(2,2-dimethyl-2-mercapto-
ethyl)(2-piperidin-1-ylethyl)amine]oxorhenium(V) (4)
unique/used=3995/3720
(Rint=0.0204)/3458,
265
parameters refined, [Dz]max/[Dz]min=1.132/−0.741 (e
−3), [D/|]max=0.025. Hydrogen atoms of C(1), C(6),
˚
The complex was prepared as described above using
ReOCl3(PPh3)2 as the precursor (0.2 mmol) and equi-
molar quantities (0.2 mmol) of 2,6-dimethyl-4-(2-pipe-
ridin-1-ylethyl)-4-aza-2,6-heptanedithiol and benzyl-
mercaptane. The complex was isolated as green–grey
crystals from a dichloromethane–ethanol solution.
Yield 42%. FT-IR (cm−1, KBr pellet): 954 (ReꢀO), 700
A
C(8), C(14) and C(15) were introduced at calculated
positions as riding on bonded atoms, the rest were
located by difference maps and refined isotropically. All
non-hydrogen atoms were refined anisotropically.
Complex 3: 2qmax=47°, scan speed 1.5° per minute,
scan range 2.35=a1a2 separation, reflections collected/
(arom).
Anal.
Calc.
for
C22H37N2OS3Re*1/2
unique/used=3412/3031
(Rint=0.0349)/2382,
269
CH3CH2OH: C, 42.44; H, 6.19; N, 4.30; S, 14.77.
Found: C, 42.75; H, 6.50; N, 4.22; S, 14.68%.
parameters refined, [Dz]max/[Dz]min=1.561/−0.916 (e
A
−3), [D/|]max=0.022. Hydrogen atoms of C(3), C(4),
˚
C(5), C(8), C(9), C(10), C(16) and C(17) were intro-
duced at calculated positions as riding on bonded
atoms, the rest were located by difference maps and
refined isotropically. All non-hydrogen atoms were
refined anisotropically.
2.6. [(4-Methyl)thiophenolato][N,N-bis(2-mercapto-
ethyl)N%,N%-diethylethylenediamine]oxorhenium-186(V)
(
186Re-3)
186ReO4− (2 mCi, 1 ml) was added to a solution of 1
Complex 4: 2qmax=49°, scan speed 4.5° per minute,
scan range 2.7=a1a2 separation, reflections collected/
ml of 0.5 M of citric acid (25 mg, 0.130 mmol) contain-
ing 1 mg of SnCl2. This solution was transferred to a
centrifuge tube containing equimolar quantities (0.02
mmol) of N,N-bis-(2-mercaptoethyl)-N%,N%-diethyl-
ethylenediamine and 4-methylthiophenol. The mixture
unique/used=4976/4532
(Rint=0.0378)/3262,
280
parameters refined, [Dz]max/[Dz]min=6.722/−0.976 (e
˚
A
−3) in the vicinity of the heavy metal, [D/|]max
=
0.036. All hydrogen atoms were introduced at calcu-

M. Pelecanou et al. / Inorganica Chimica Acta 287 (1999) 142–151
145
Table 1
Summary of crystal, intensity collection and refinement data for complexes 2, 3, and 4
2
3
4
Formula
C15H24NO2S4Re
564.80
C17H29N2OS3Re
559.81
13.188(1)
7.542(1)
C
650.98
16.720(6)
11.013(3)
16.747(5)
23H40N2O1.5S3Re
Formula weight
˚
a (A)
9.610(2)
9.628(2)
11.791(2)
108.307(8)
67.735(6)
90.166(8)
950.04
˚
b (A)
˚
c (A)
21.193(1)
h (°)
i (°)
k (°)
103.260(2)
113.44(1)
3
˚
V (A )
2051.74
4
1.812/1.80
P21/c
2829.08
4
1.528/1.50
P21/c
Z
D
2
calc/Dmeas (Mg m−3
)
1.974/1.96
(
Space group
Temperature (K)
P1
296
296
296
˚
Radiation u (A)
Mo Ka 0.71073
65.70
Mo Ka 0.71073
59.90
Mo Ka 0.71073
43.50
Absorption coefficient (v) (cm−1
)
Octants collected
9h, −k, 9l
g=0.0001
1.39a
9h, −k, l
unit weights
4.21b
9h, k, −l
g=0.0002
3.73c
w=1/|2(Fo)+gFo2
sig=[ꢀw(ꢁFoꢁ−ꢁFcꢁ)2/(N−P)]1/2
R=ꢀꢂFoꢁ−ꢁFcꢂ/ꢀꢁFoꢁ
0.0235a
0.0392b
0.0454b
0.0732c
0.0950c
Rw=(ꢀw(ꢁFoꢁ−ꢁFcꢁ)2/ꢀwꢁFoꢁ )
0.0336a
2 1/2
a For 3458 reflections with Fo\6|(Fo).
b For 2382 reflections with Fo\3.6|(Fo).
c For 3262 reflections with Fo\6|(Fo).
lated positions as riding on bonded atoms. All non-hy-
drogen atoms were refined anisotropically. The ethano-
lic solvent molecule diffused during data collection and
was refined isotropically with occupation factors fixed
at 10.50.
Complex 3: For C-1/C-4, TC=7°C (280.0 K), Dw in
the absence of exchange=42.8 Hz. For C-2/C-3, TC=
16.5°C (289.5 K), Dw in the absence of exchange=
100.2 Hz. Both exchanging pairs give DG"c values in
complete agreement with each other and, since the
difference in coalescence temperature is small, the aver-
age DG"c is reported as representative of the complex.
Complex 4: For C-7/C-9, TC= −17.0°C (256.0 K),
Dw in the absence of exchange=11.2 Hz, and for the
corresponding protons on C-7/C-9, TC= −9.5°C
(263.5 K), Dw in the absence of exchange=18.3 Hz.
Both 13C and 1H data give DG"c data in complete
agreement with each other and, since the coalescence
temperature is not appreciably affected in going from
2.8. Electrochemistry
Voltammograms were recorded on a PAR model
174A polarographic analyzer operated in conjuction
with a model 175 Universal programmer. An undivided
three-electrode cell was used with a Pt working elec-
trode (0.2 cm2), an Ag wire as the reference and a Pt
wire as the counter electrode.
1
13C to H, the average DG"c is reported as representa-
tive of the complex.
2.9. NMR studies
1H (250.13 MHz) and 13C (62.90 MHz) NMR spectra
were recorded on a Bruker AC 250E spectrometer.
Samples were prepared in CDCl3. Chemical shifts were
relative to TMS. Spectral parameters have been previ-
ously reported in the literature [7b,c,d,e].
3. Results and discussion
Complexes 1–4 are stable in the solid phase and in
solution. They are less soluble in CHCl3 than their
technetium analogues. They give green (complexes 1–3)
to green–grey (complex 4) crystals as opposed to the
technetium complexes that give dark red (complexes
1%–3%) to brown (complex 4%) crystals.
The ReꢀO stretch for complexes 1, 2, 3, and 4 occurs
at 964, 945, 939, and 954 cm−1, respectively. These
values are well within the range of the ReꢀO stretch
reported in the literature for some very well character-
2.9.1. Calculation of ZG"
The coalescence region for the resonances under in-
vestigation was initially determined by obtaining spec-
tra at 5°C intervals. Spectra were then obtained in the
coalescence region at 0.5°C intervals in order to deter-
mine the coalesence temperature (TC).

146
M. Pelecanou et al. / Inorganica Chimica Acta 287 (1999) 142–151
Fig. 2. ORTEP diagram of complex 3 with 50% thermal probability
ellipsoids showing atomic labeling scheme.
Fig. 1. ORTEP diagram of complex 2 with 50% thermal probability
ellipsoids showing atomic labeling scheme.
~=1 for a perfect trigonal bipyramid) which in our
complexes is 0.66, 0.63 and 0.48 for 2, 3, and 4,
respectively.
ized ReꢀO complexes [5]. The TcꢀO stretch of the
corresponding complexes 1%, 2%, 3%, and 4% occurs at 943
[7a], 920 [7a], 910 [7f] and 934 [7c] cm−1, respectively.
This difference in oxometal stretching frequency of ca.
20 cm−1 between analogous ReO and TcO complexes
has been reported in the literature for other ligand
systems like diaminedithiol [5a,b,e] and diamidedithiol
[5f] and has been attributed to the greater orbital
overlap of the 5d orbitals of rhenium compared to the
4d orbitals of technetium [5i].
The ReꢀO, Re–S and Re–N bond distances in 2, 3,
and 4 were found in the ranges observed in analogous
oxorhenium and oxotechnetium complexes. The ReꢀO
˚
bond distance is ca. 1.68 A. The Re–S bond distances
˚
in 2 and 3 range from 2.271(3) to 2.280(1) A for the
3.1. Crystallography
ORTEP diagrams of complexes 2, 3 and 4 are given in
Figs. 1–3, respectively. Selected bond distances and
angles for the three complexes are given in Table 2. All
three complexes have the syn configuration with the
side chain on the coordinated nitrogen cis to the oxo
group. The oxorhenium moiety is coordinated to a
tridentate ligand having an SNS donor set and to a
monodentate thiol. The coordination geometry about
rhenium(V) in complexes 2 and 3 is distorted trigonal
bipyramidal with the sulfur atoms of the tridentate
ligand and the oxo group occupying the basal plane
while the nitrogen atom of the SNS ligand and the thiol
˚
are directed at the apices. Rhenium lies 0.096 A (for 2)
˚
and 0.082 A (for 3) out of the basal plane toward the
monodentate thiol. In the case of complex 4 the coordi-
nation sphere can be described as trigonally distorted
square pyramidal with the oxo group at the apex. This
different description is a result of the calculation of the
trigonality index ~ (~=0 for a perfect square pyramid,
Fig. 3. ORTEP diagram of complex 4 with 50% thermal probability
ellipsoids showing atomic labeling scheme.

M. Pelecanou et al. / Inorganica Chimica Acta 287 (1999) 142–151
147
Table 2
Selected bond distances (A) and angles (°) for complexes 2, 3, and 4
C(4)–S(2) are 48.10 and 52.85° for 2 and 47.95 and
51.52° for 3. These angles are different in 4 (−45.54
and 40.06°, respectively) as a result of the different
orientation of the tridentate ligand about the oxorhe-
nium group. The piperidine ring in 4, exists in the
stable chair form with C(15) and C(12) being 0.70 and
˚
2
3
4
˚
Bond distances (A)
Re–S(1)
Re–S(2)
Re–S(3)
Re–N(1)
Re–O(1)
2.274(1)
2.280(1)
2.305(1)
2.215(3)
1.698(3)
2.271(3)
2.278(3)
2.312(3)
2.212(8)
1.678(8)
2.261(4)
2.273(5)
2.259(4)
2.24(1)
˚
0.61 A above and below the mean plane defined by the
other four atoms.
1.68(1)
3.2. Electrochemistry of 3 and 3%
Bond angles (°)
S(1)–Re–S(2)
S(1)–Re–S(3)
S(2)–Re–S(3)
S(1)–Re–N(1)
S(2)–Re–N(1)
S(3)–Re–N(1)
S(1)–Re–O(1)
S(2)–Re–O(1)
S(3)–Re–O(1)
N(1)–Re–O(1)
121.0(1)
90.0(1)
85.0(1)
84.1(1)
83.1(1)
160.8(1)
118.6(1)
119.3(1)
103.8(1)
95.1(1)
121.0(1)
87.0(1)
85.5(1)
83.6(2)
83.3(3)
159.0(3)
120.1(3)
118.4(3)
104.6(3)
96.3(4)
127.5(2)
90.1(1)
84.3(2)
82.9(3)
82.1(3)
156.2(3)
115.6(5)
116.5(5)
103.0(5)
100.5(5)
The electrochemical behavior of complex 3 and of its
technetium analogue 3% was studied in CH3CN at r.t.
The cyclic voltammetric data are shown in Fig. 4. Both
complexes give rise to (at potential scan rates ranging
from 50 to 500 mV s−1) voltammograms of little
interest with no apparently reversible systems present.
Lack of reversibility has been also reported for oxo-
technetium complexes with tetradentate Schiff base lig-
ands [15]. The shape of the voltammograms is quite
similar for both complexes. In the anodic region, com-
plex 3 displays peaks at 1.01 and 1.33 V while complex
3% displays peaks at 1.00 and 1.41 V. On the reverse
scans no cathodic waves were observed even at sweep
rates of up to 500 mV s−1. The absence of the reverse
electron-transfer steps indicates that the oxidized spe-
cies are unstable.
bonds in the basal plane, while a reasonable lengthen-
ing is observed for the Re–S apical bonds (2.305(1) and
˚
2.312(3) A for 2 and 3, respectively). In the analogous
oxotechnetium complexes 2% and 3% the Tc–S bond
lengths observed are in the same ranges. The Re–N
bond distance is ca. 2.21 A in both 2 and 3 a fact that
is consistent with the Tc–N bond lengths found in 2%
and 3%. In complex 4, the Re–S bond lengths range
˚
In the negative potential region complex 3 shows two
ill-developed waves at −0.53 and −0.64 V that could
be due to reduction to the Re(IV) and Re(III) states.
The technetium complex 3% shows reduction waves at
˚
from 2.259(4) to 2.273(5) A, as in the case of the
analogous oxotechnetium complex 4%. The Re–N bond
˚
distance (2.24(1) A) is slightly longer than usual (2.0–
˚
2.21 A) [14] as was also found in 4% [7c]. The bond
angles between the atoms of the basal plane of the
trigonal bipyramidal complexes 2 and 3 are close to the
ideal value of 120° (118.4(3)–121.0(1)°) but the S(3)–
Re–N(1) angles are away from 180° (160.8(1) and
159.0(3)° for 2 and 3, respectively) showing a degree of
distortion. In the case of the trigonally distorted square
pyramidal complex 4, the angles between the opposite
atoms of the basal plane are 156.2(3) and 127.5(2)° for
S(3)–Re–N(1) and S(1)–Re–S(2), respectively.
The two five-membered rings defined by the atoms
Re, S(1), C(1), C(2), N(1) and Re, S(2), C(3), C(4), N(1)
exist in the envelope form. In 2 and 3, atoms C(2) and
˚
C(3) adjacent to nitrogen are ca. 0.60 and ca. 0.65 A
displaced from the mean plane defined by the remain-
ing four. This seems to be the case in a series of
analogous oxotechnetium and oxorhenium complexes
showing distorted trigonal bipyramidal geometry. But
in complex 4, as in its oxotechnetium analogue 4%,
atoms C(2) (which is bonded to the coordinated nitro-
gen) and C(4) (which carries two methyl substituents)
are 0.55 and 0.62 A displaced from the mean plane of
the other four atoms. The torsion angles of the triden-
tate backbone S(1)–C(1)–C(2)–N(1) and N(1)–C(3)–
Fig. 4. Cyclic voltammograms of (A) 1 mM of complex 3 in 0.1 M
Et4NClO4/CH3CN and (B) 1 mM of complex 3% in 0.1 M Et4NClO4/
CH3CN. Scan rate 100 mV s−1. At a Pt working electrode and an Ag
pseudo reference.
˚

148
M. Pelecanou et al. / Inorganica Chimica Acta 287 (1999) 142–151
−0.18 and −0.43 V. These data are in agreement with
the existing literature data according to which tech-
netium complexes are easier to reduce than their rhe-
nium analogues with the DE°% (Tc–Re) ranging from
150 to 320 mV for neutral complexes [16]. No anodic
waves were observed on the reverse scan suggesting that
reductions proceed essentially with decomposition of
the complexes.
Electrochemical studies of oxotechnetiun and ox-
orhenium complexes with bidentate ON Schiff-base or
8-quinolinol ligands [17] as well as tridentate ONN
Schiff-base ligands [5g] performed within the voltage
range (+1.6 to −0.7 V) used in our study show peaks
that are not observed with our complexes. Apparently,
the ligating system is determinant in the access and
stability of the various oxidation states in these
elements.
Chemical exchange occurs between positions 1 and 4
as well as between 2 and 3 due to the conformational
mobility of the S1–C1–C2–N–C3–C4–S2 chelated
ligand backbone. Temperature studies were conducted
in the range of −50 to 55°C. At r.t. (25°C) exchange
takes place at intermediate rates on the NMR time
scale and the resonances of protons and carbons of the
S1–C1–C2–N–C3–C4–S2 chelated moieties of com-
plexes 2, 3, and 4 as well as of the benzyl group of
complex 4 appear broad. On raising the temperature,
the spectra become simpler; the increase in the rate of
fluxional mobility of the ligand generates an effective
plane of symmetry passing through the Re–O–N
atoms so that the S1–C1–C2–N and S2–C4–C3–N
parts of the chelated backbone become magnetically
equivalent. On lowering the temperature, conforma-
tional motion becomes slow and the exchanging pro-
tons and carbons on S1–C1–C2–N and S2–C4–
C3–N appear as separate spin systems. Fig. 5 displays
the temperature dependence of the 13C spectrum of
complex 3 while Fig. 6 displays the temperature depen-
3.3. NMR studies
1H and 13C NMR chemical shifts for complexes 1–4
in CDCl3 at 25°C are reported in Tables 3 and 4. The
chemical shifts of the corresponding oxotechnetium
complexes (1%–4%) are included in the tables for com-
parison purposes.
The four complexes display the general characteris-
tics of the class of ‘3+1’ mixed ligand complexes
with the side chain on nitrogen directed towards the
oxygen of the ReꢀO core (syn configuration), as pre-
sented in a series of publications from this laboratory
[7b,c,e,f].
1
dence of the H spectrum of complex 4.
The DG"c for the conformational inversion of the
chelated S1–C1–C2–N–C3–C4–S2 ligand backbone
was determined for complexes 3 and 4 in CDCl3 [18]. In
the case of complex 3 calculation was based on the
temperature dependence of the resonances of the C1/C4
and of the C2/C3 exchanging carbons (Fig. 5). In the
case of complex 4 calculation was based on the temper-
1
ature dependence of the H and 13C resonances of the
exchanging C-7/C-9 methyl groups. From the chemical
Table 3
1H (lC¸ , ppm) chemical shifts for complexes 1–4a and their corresponding technetium analogues 1%–4% in CDCl3 at 25°C
1
1%b
2
2%b
3
3%c
4
4%d
H-1 (H-4)endo
H-1 (H-4)exo
H-2 (H-3) endo
H-2 (H-3) exo
H-5
H-6
H-7
H-8
H-9
3.60
2.83
3.36
3.14
3.80
2.92
2.61
1.30
3.69
3.10
3.65
3.29
3.87
2.95
2.62
1.31
3.59
2.90
3.33
2.71
4.01
2.95
2.62
1.30
3.57
2.99
3.49
2.72
4.09
2.98
2.64
1.31
3.60
2.86
3.44
2.83
3.92
2.87
2.58
1.06
2.58
1.06
3.59
2.98
3.63
2.82
4.02
2.89
2.58
1.07
2.58
1.07
3.86
2.24
4.05
2.84
1.83
1.65
1.83
1.65
2.46
1.59
1.48
4.87
7.41
7.29
7.17
3.90
2.24
4.17
2.85
1.89
1.52
1.89
1.52
2.49
1.60
1.60
4.94
7.42
7.28
7.18
H-10
H-11 (H-15)
H-12 (H-14)
H-13
H-16
H-2% (H-6%)
H-3% (H-5%)
H-4%
7.55
6.94
7.56
6.92
7.53
7.20
7.52
7.19
H-7%
3.84
3.84
2.40
2.39
a Numbering of the atoms is shown in Scheme 1.
b Data from Ref. [7b].
c Data from Ref. [7f].
d Data from Ref. [7c].

M. Pelecanou et al. / Inorganica Chimica Acta 287 (1999) 142–151
149
Table 4
13C (lC, ppm) chemical shifts for complexes 1–4a and their corresponding technetium analogues 1%–4% in CDCl3 at 25°C
1
1%b
2
2%b
3
3%c
4
4%d
C-1 (C-4)
C-2 (C-3)
C-5
C-6
C-7
C-8
C-9
C-10
C-11 (C-15)
C-12 (C-14)
C-13
C-16
C-1%
C-2%(C-6%)
C-3%(C-5%)
C-4%
43.42
65.41
66.68
25.56
26.54
14.60
37.25
63.05
65.26
25.49
26.64
14.63
41.79
62.96
63.47
25.32
26.45
14.64
36.73
61.17
62.15
25.17
26.46
14.66
42.07
62.91
61.36
48.16
46.94
11.74
46.94
11.74
36.98
61.58
76.31
61.02
54.31
30.74
31.19
30.74
31.19
55.05
26.02
24.03
49.28
142.07
129.28
128.24
126.38
56.66
75.15
60.04
53.97
31.11
31.55
31.11
31.55
55.08
26.06
24.07
44.30
141.65
129.22
128.29
126.42
61.33
59.98
47.90
47.01
11.67
47.01
11.67
145.04
134.50
113.58
158.54
55.25
138.25
135.11
113.45
158.91
55.26
150.05
133.27
128.79
136.31
21.16
143.46
133.75
128.69
136.86
21.33
C-7%
a Numbering of the atoms is shown in Scheme 1.
b Data from Ref. [7b].
c Data from our laboratory.
d Data from Ref. [7c].
shift difference of the exchanging peaks in the absence
of exchange and from the coalescence temperature,
DG"c was calculated to be 13.890.1 kcal mol−1 for
complex 3 and 13.490.1 kcal mol−1 for complex 4.
These values are in agreement with the value of 13.79
0.1 kcal mol−1 reported from our laboratory for the
fluxional motion of the syn isomer of the ReO-
[(C2H5)2NCH2CH2N(CH2CH2S)2][SC6H4OCH3] [7e] in-
dicating that minor structural changes such as the
presence of the methyl groups on the ligand backbone,
different substituents on the N-side chain, or different
para-substituents on the aromatic thiol do not appre-
ciably affect the mobility of the ligand. On the con-
trary, the replacement of the aromatic thiol coligand by
a smaller group makes the complex more flexible as was
evidenced in the case of complex 1 where a chlorine
1
atom is in the place of the aromatic thiol. At r.t., H
and 13C resonances for complex 1 are sharp and no
significant change is observed in the spectra down to
−55°C.
The chemical shifts of geminal protons of the S1–
C1–C2–N and S2–C4–C3–N chelated backbone are
differentiated according to their vicinity to the ReꢀO
core. Protons pointing towards the oxygen (endo pro-
tons) appear at higher frequencies than those pointing
away from the oxygen (exo protons). In the present
study, the observed differences between endo and exo
protons range from 1.6 (protons on C-2/C-3 of complex
4) to 0.2 ppm (protons on C-2/C-3 of complex 1).
The aromatic ring substituent has little effect on the
chemical shifts of protons and carbons of the chelated
ligand backbone, since variation of the para-substituent
Scheme 1.

150
M. Pelecanou et al. / Inorganica Chimica Acta 287 (1999) 142–151
from CH3 (complex 3) to OCH3 [7e] to NO2 [19]
produces no significant changes in the chemical shifts of
the ligand backbone atoms. This fact indicates that
electronic properties of the ring are not transmitted to
the chelated part of the ligand through the metal core
in this type of complex.
OC6H4S)] complex and was found to be 1.9 kcal mol−1
lower than that of the rhenium analogue [7e].
4. Concluding remarks
The data in Table 3 show that coordinated ligand
protons appear at very similar chemical shifts in
analogous ReꢀO and TcꢀO complexes. The observed
differences DlH (Re–Tc) range from90.01 to 0.30
ppm and cannot be associated with specific positions in
the molecule. Lack of metal dependence of the proton
NMR chemical shifts has been observed in various
classes of homologous technetium and rhenium com-
plexes [6]. On the contrary, the 13C data in Table 4
show metal dependence. Specifically, carbons close to
the metal core appear more deshielded in the rhenium
complexes compared to the technetium. The difference
DlC (Re–Tc) for the carbons that are connected to the
metal through a sulfur atom (carbons C-1/C-4 and C-1%
in complexes 2 and 3) ranges from 4.9 to 6.8 ppm and
is higher than the difference of 1.0 to 2.4 ppm observed
for the carbons connected to the metal through a
nitrogen atom (carbons C-2/C-3 and C-5). Similar dif-
ferences are present in the 13C data reported for ho-
mologous technetium and rhenium diaminedithiol [5b]
and diamidedithiol [5c] complexes.
We have shown that the action of the tridentate
SN(R)S aminodithiol ligand on a suitable ReO3+ pre-
cursor in the presence of a monodentate ligand gives
stable, neutral and lipophilic oxorhenium complexes of
the ‘3+1’ type. The complexes formed have the same
structure as those formed with oxotechnetium and the
same ligand system. Analogous oxorhenium and oxo-
technetium complexes display similar chemical charac-
teristics with
a few exceptions. Differences were
observed in the frequency of the oxometal stretch (ca.
20 cm−1 higher for ReꢀO than TcꢀO), the NMR
chemical shifts of carbons close to the metal core
(rhenium has a deshielding effect relative to technetium)
as well as in the fluxional mobility of the ligand back-
bone (rhenium complexes are less flexible than their
As has been previously reported [7e], technetium
complexes of the ‘3+1’ type are more flexible than
their rhenium analogues giving at r.t. sharp, motionally
averaged NMR spectra. The DG"c has been calculated
for the TcO[(C2H5)2NCH2CH2N(CH2CH2S)2][(p-CH3-
Fig. 6. 1H NMR spectra (range lH 5.30–2.64) of complex 4 at
different temperatures. The effect of the fluxional motion of the
ligand on the appearance of the benzylic protons (H-16) is clearly
displayed. At −50°C the diastereotopic benzylic protons appear as
two separate doublet peaks. As the temperature is raised the two
peaks broaden and at 25°C they have coalesced into one broad peak.
At 50°C one sharp average peak characteristic of fast exchange is
present. The rest of the peaks in the spectra are similarly affected by
the exchange phenomena.
Fig. 5. 13C NMR spectra (range lC 68.1–38.7) of complex 3. At
−20°C exchanging carbons C-1/C-4 and C-2/C-3 appear as separate
peaks. As the temperature is raised the peaks coalesce (7°C is the
coalesence temperature for C-1 and C-4) and finally give rise to sharp
average peaks.

M. Pelecanou et al. / Inorganica Chimica Acta 287 (1999) 142–151
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technetium analogues). In addition, in the electrochemi-
cal study the oxorhenium complex 3 proved harder to
reduce than its oxotechnetium analogue 3%.
It was shown for complex 3 that the macroscopic
chemistry is successfully transferred to the 186Re tracer
level in the same way that the 99Tc chemistry is trans-
ferred to the 99mTc level [7g]. Preliminary biodistribu-
tion studies show that complex 186Re-3 has a similar
biodistribution pattern to the 99mTc-3% complex [20].
Our findings suggest that mixed ligand systems can
serve as the basis for the development of target-specific
radiopharmaceuticals (186Re and 188Re) for therapeutic
purposes.
5. Supplementary material
Tables of crystal data, fractional atomic coordinates
and anisotropic thermal parameters, full listings of
bond distances and angles (14 pages) are available from
the author on request.
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Acknowledgements
We thank Dr D. Petridis of the Institute of Materials
Science, NCSR Demokritos, for the electrochemical
study. A. Terzis is grateful to Mr John Boutaris and the
Agricultural Bank of Greece (ATE) for financial
support.
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